Fast neutron nuclear reactors generally comprise a concrete structure including a cylindrical vessel well with a vertical axis, inside which is located the main reactor vessel surrounded by the safety vessel and closed by the slab resting on the upper part of the vessel well. The main vessel and the safety vessel of the reactor are suspended from the lower part of the slab, which consists of a composite structure of steel and concrete.
This structure consists of an annular envelope filled with concrete, providing, in its central part, a cylindrical space to enable the slab to accommodate the large rotating plug carrying all the devices for handling the fuel assemblies forming the reactor core located inside the vessel.
The annular envelope consists of two coaxial cylindrical sleeves and of two annular face plates joined to each of the sleeves in their upper part and lower part, respectively.
Cylindrical spaces are provided inside the envelope by sleeves passing right through the slab and emerging in the region of circular openings provided in the upper and lower face-plates of the slab. The concrete filling occupies the interior volume of the envelope except for these cylindrical spaces permitting the passage of the reactor components immersed in the vessel by their lower parts, such as the primary pumps and the intermediate exchangers.
Radially oriented stiffeners permit rigid assembly of the cylindrical sleeves, the upper and lower face plates and the sleeves for the passage of the components. These various elements are welded together.
The outer cylindrical sleeve of the envelope of the slab is used for fixing the latter on top of the vessel well. In fact, a sleeve of the same diameter is fixed to the upper part of the vessel well, and the slab is fixed by joining the outer sleeve of the slab to this support sleeve.
The purpose of the reactor vessel is to contain the primary fluid kept at high temperature by the heat evolved by the core. This primary fluid is generally liquid sodium.
Those parts of the slab oriented towards the inside of the vessel, i.e., the lower face-plate and the sheaths for the passage of the components, must therefore be cooled in order to limit the extent to which they are heated and in order to prevent a reduction in their mechanical strength. Cooling tubes are therefore placed in contact with the lower face plate and the sheaths for the passage of components, and water circulation is set up inside these cooling tubes, the latter being embedded in the filling concrete of the slab.
Moreover, the lower face plate is protected from the heat of the primary fluid by a lagging.
The expansions of the slab are therefore limited, and it is thus possible to join it to the support sleeve fixed to the vessel well.
When the large rotating plug is resting in its housing in the center of the annular slab, its extremely large weight is entirely supported by the slab, of which the upper face plate operates under compression and the lower face plate under tension.
The whole of the metal structure of the slab, comprising the outer sleeve, the inner sleeve, the upper and lower face plates, the sheaths for the passage of the components and the radially oriented, vertical stiffeners, must therefore possess very great strength and rigidity.
In particular, to transmit the shear forces without discontinuity, the stiffeners must be very close to one another, whereby the interior volume of the envelope of the slab is divided up into extremely small portions. The filling concrete therefore makes virtually no contribution to the strength of the whole and serves only to provide protection against the radiation from the materials contained inside the vessel.
The metal structure of the slab is extremely heavy, expensive and difficult to weld, this structure also being made more complex by the fact that it is necessary to provide cooling tubes in contact with the sheaths for the passage of the components and in contact with the lower face plate, which withstands very large stresses. These cooling tubes have to pass through the radial stiffeners, in which numerous openings must be provided.